When he had put by the ten francs for his and Gervaise's share of the feast--the two children not being charged for--he had exactly six francs left--the price of a low mass at the altar of the poor.

He had no liking for those black crows, the priests.
It would gripe him to pay his last six francs to keep their whistles wet; however, a marriage without a mass wasn't a real marriage at all.
Going to the church himself, he bargained for a whole hour with a little old priest in a dirty cassock who was as sharp at dealing as a push-cart peddler.

Coupeau felt like boxing his ears.

For a joke, he asked the priest if he didn't have a second-hand mass that would do for a modest young couple.

The priest, mumbling that God would take small pleasure in blessing their union, finally let his have his mass for five francs.
Well after all, that meant twenty sous saved.
Gervaise also wanted to look decent.